1. Field of the Invention
The invention relates to junction-type field-effect transistors, especially but not exclusively very sensitive field-effect transistors which operate on low supply voltages with low threshold voltages and which may be used in particular as input elements for low noise level operational amplifiers, for example the amplifiers installed in sensors for medical uses. The invention also relates to the manufacture of such transistors. Experience has shown that, at present, very sensitive transistors of this type can only be made satisfactorily, at least as far as their channel and gate regions are concerned, by means of ion implantation. To obtain a very high sensitivity the channel and gate regions (particularly the channel region) must in fact be extremely thin and lightly doped; it is almost impossible to do this, in a reproducible fashion, other than by implantation.
2. Description of the Prior Art.
A known junction-type field-effect transistor comprises, adjacent to the surface of a semiconductor layer of the first conductivity type,
a first, highly doped, source region of the second conductivity type, PA1 a second, highly doped, drain region of the second conductivity type and spaced from the source region, PA1 a third, gate region of the first conductivity type extending between the first and second regions and in ohmic contact with said layer, and PA1 a fourth, lightly doped, channel region of the second conductivity type, located below the third region and also extending between the first and second regions. This transistor structure, which can be created using implantation, can be incorporated in an integrated circuit also comprising further similar transistors and other transistors of the bipolar type.
In the known transistors, the third region (gate region) is, according to the specific type, either connected or not connected to the semiconductor layer, the deep part of which situated below the channel region, cooperates with the gate region. This connection is often made, particularly when it is a case of improving the sensitivity of the transistor; the semiconductor layer and the third region are then always at the same potential.
To bring about this connection, the third region projects in relation to the subjacent channel region in such a way that it makes contact laterally with the semiconductor layer. This is shown, for example, in French Pat. No. 1 431 642, which is concerned with a field-effect current limiter in which the upper control region (gate) of N-type conductivity is diffused in a larger diffused region of P-type conductivity which includes the source, substrate and channel. The gate region, with a rectangular surface, extends in length along both sides of the P-type region and then comes into contact with the material of N-type conductivity in which the device has been created and a deep part of which constitutes the lower control region.
To obtain the desired connection between the gate region and the semiconductor layer of the same conductivity type in such a field-effect transistor created using implantation, the channel region is created in a first implantation operation and through a first mask, then the gate region is created in a second implantation operation and through a second mask of dimensions greater than those of the first mask.
These masking operations are in addition to those required for the production of bipolar transistors in an integrated circuit comprising the two types of transistors. They inevitably place a burden on the manufacture of such an integrated circuit.